Integrated wavelength multi/demultiplexers are important components for wavelength division multiplexing (WDM) optical communication systems. Integration offers the advantages of compactness, reliability, and reduced packaging costs. Further, implementation in a semiconductor material, particularly the InGaAsP/InP system important for optical fiber communications systems, would permit monolithic integration of these passive devices with active ones, such as lasers, modulators, optical switches, and detectors, resulting in sophisticated wavelength sensitive photonic integrated circuits with complex functionalities.
Heretofore, one of the major drawbacks in an integrated wavelength multi/demultiplexer is the polarization sensitivity of the device. Since an optical signal propagating through an optical fiber has an indeterminate polarization state, the switching/routing devices must be substantially polarization insensitive. However, planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm. As WDM systems are being designed towards smaller and smaller channel spacing (from 1.6 nm to 0.8 nm or even less in the future), even a small polarization dependent wavelength shift (e.g. 0.3-0.4 nm) is of concern.
Two types of integrated wavelength multi/demultiplexers that have been widely investigated are phased waveguide arrays and grating-on-a-chip spectrometers.
Grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is usually much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of teeth in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can be scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to compensate for the undersized polarization sensitivity; for example the insertion of a half wave plate in the middle of the waveguides array is described by H. Takahashi, Y. Hibino, and I. Nishi, in a paper entitled "Polarization-insensitive arrayed waveguide grating wavelength multiplexer on silicon", Opt. Lett., vol. 17, no. 7, pp. 499-501, 1992.
Alternatively, the use of non-birefringent waveguides with a square cross section has been described by J. B. D. Soole, M. R. Amersfoort, H. P. Leblanc, N. C. Andreadakis, A. Raijhel, C. Caneau, M. A. Koza, R. Bhat, C. Youtsey, and I. Adesida, in a paper entitled "Polarization-independent InP arrayed waveguide filter using square cross-section waveguides", Electron. Lett., vol. 32, pp. 323-324, 1996.
Birefringence compensation using two different rib waveguides has been described by P. C. Chou, C. H. Joynerm M. Zirngibl, in U.S. Pat. No. 5,623,571 entitled "Polarization compensated waveguide grating router". In the '571 patent the polarization compensation is not within the slab waveguiding region. This technique requires either two regrowth steps as described in the patent and in a paper by the same authors entitled "Polarization compensated waveguide grating router on InP", Electron. Lett., vol. 31, pp. 1662-1664, 1995, or two etching steps as described by C. G. M. Vreeburg, C. G. P. Herben, X. J. M. Leijtens, M. K. Smit, F. H. Groen, J. J. G. M. van der Tol and P. Demeester, in a paper entitled "An improved technology for eliminating polarization dispersion in integrated phasar demultiplexers", in Proc. 23.sup.rd Conf. on Optical Comm. (ECOC'97), pp. 3.83-3.86, Edinburgh, UK, 1997. In addition to increased complexity in fabrication process, the reduced cladding layer thickness in the polarization compensating rib/ridge waveguides resulted in a reduced lateral index contrast, and consequently increased phase errors due to enhanced coupling between adjacent waveguides. In order to avoid radiation loss due to reduced index contrast, the polarization compensating waveguides need to be implemented in straight waveguide section, which leads to an additional straight section length of the arrayed waveguides and consequently a larger device size.
Yet another alternative in the attempt to overcome polarization sensitivity is dispersion matching with adjacent diffraction orders which has been described by M. Zirngibl, C. H. Joyner, L. W. Stulz, Th. Gaigge and C. Dragone, in a paper entitled "Polarization independent 8.times.8 waveguide grating multiplexer on InP", Electron. Lett., vol. 29, pp. 201-201, 1993, and by L. H. Spiekman, M. R. Amersfoort, A. H. de Vreede, F. P. G. M. van Ham, A. Kuntze, J. W. Pedersen, P. Demeester, and M. K. Smit, in a paper entitled "Design and realization of polarization independent phased array wavelength demultiplexers using different array order for TE and TM", J. Lightwave Technol., vol. 14, pp. 991-995, 1996.
Another approach is that of using layer structures with low birefringence by using thick guiding layer and low refractive index contrasts has been described by H. Bissessur, F. Gaborit, B. Martin, P. Pagnod-Rossiaux, J. L. Peyre and M. Renaud, in a paper entitled "16 channel phased array wavelength demultiplexer on InP with low polarization sensitivity", Electron. Lett., vol. 30, pp. 336-337, 1994.
For diffraction grating based wavelength multi/demultiplexers, only the last two approaches are possible. In the polarization compensation method which attempts to match the TE and TM passband to two adjacent diffraction orders, the free spectral range (FSR) of the grating needs to be chosen equal to the wavelength split between the two modes. In this case, the passband corresponding to the mth-order for TE will overlap with the (m-1)th order for TM. A severe drawback of this method is that the available FSR for WDM channels is limited by the polarization split, which is determined by the waveguide layer structure. It is usually limited to a few nanometers. A large polarization split is preferable in this case. In addition, since the polarization dispersion is very sensitive to the exact layer composition and thickness, it is difficult to obtain a good match due to the non-uniformity and non-reproducibility of wafer growths.
Another method for achieving polarization insensitive operation in diffraction grating based wavelength multi/demultiplexer is to use a birefringence-reduced layer structure, combined with an input/output waveguide design for a flattened channel response. Polarization dispersion as small as 0.3-0.4nm has been obtained with InGaAsP/InP double heterostructures as is described by J.-J. He, B. Lamontagne, A. Delage, L. Erickson, M. Davies, and E. S. Koteles, in a paper entitled "Monolithic integrated wavelength demultiplexer based on a waveguide Rowland circle grating in InGaAsP/InP", J. Lightwave Tech, vol. 16, pp.631-638, 1998. Lower birefringence waveguides can be designed by using a thick guiding layer and low refractive index contrast between the guiding and cladding layers. However, low index contrast InGaAsP/InP layers are very difficult to grow in practice. One way to obtain low index contrast waveguides is to use homogenous InP with different doping levels for the guiding and cladding layers, as suggested by Gini, W. Hunziker, and H. Melchior, in a paper entitled "Polarization independent WDM multiplexer/demultiplexer module", J. Lightwave Tech, vol. 16, pp.625-630, 1998. Although a polarization dispersion as small as 0.001 nm was obtained, the layer structure design poses severe limitations on what kind of devices can be integrated. Moreover, the thick guiding layer results in a much more stringent requirement on the verticality of the grating facet in order to keep reflection loss low.
It is an object of the invention to provide a compact, diffraction grating or phased array based optical multiplexer/demultiplexer that is substantially polarization insensitive and which overcomes many of the limitations of prior art devices.